In some recording medium playing apparatuses such as a video disk player or the like in order to rapidly move a pickup carrying and supporting carriage, a linear motor is used to drive the carriage.
FIG. 5 is a partially sectional perspective view illustrating an example of a carriage driven by a linear motor, in which a pickup 1 for optically reading a recorded signal from a track of a disk (not shown) is supported and carried by a carriage 2. The carriage 2 is arranged to move along a pair of guide rails 3a and 3b extending in the radial direction of the disk.
A driving coil 11 and velocity detecting coil 12 are fixedly mounted on opposed ends of the carriage 2. The driving coil 11 has an iron core constituted by a center yoke of a magnetic circuit 13 disposed parallel to the guide rail 3a with the driving coil 11 being movable along the center yoke. The magnetic circuit 13 includes a pair of permanent magnets disposed, for example, inside the yoke in opposition to each other with the driving coil 11 interposed therebetween. Thus, an electromagnetic force in the forward or backward direction (depending on the direction of the driving current) is generated when the driving current is supplied to the driving coil 11. The velocity detecting coil 12 moves along a yoke of a magnetic circuit 14 extending parallel to the guide rail 3b, the coil 12 interlinking with the flux of the magnetic circuit 14 so as to obtain an electromotive force corresponding to its velocity of movement. This electromotive force is used for velocity control purposes and the like as a velocity detection output. The driving coil 11 and the magnetic circuit 13 constitute a linear motor 10. The magnetic circuit 13 and the magnetic circuit 14 constitute magnetic paths.
The linear motor 10 is driven by a driving circuit such as shown in FIG. 6.
In FIG. 6, a control signal is supplied from a carriage movement control circuit (not shown) to a driving amplifier 22 through a subtractor 21. The control signal is power-amplified by the driving amplifier 22 and supplied to the driving coil 11 as a driving current. When the driving current is supplied to the driving coil 11, an electromagnetic force corresponding to the level of the driving current is generated so as to move the carriage 2. With the movement of the carriage 2, an electromotive force is generated in the velocity detecting coil 12. The above-mentioned electromotive force corresponding to the velocity of the carriage 2 is amplified by an amplifier 23 so as to provide a velocity output having a proper level, the velocity output being supplied to a negative input terminal of the subtractor 21 through a velocity equalizer 24. The velocity equalizer 24 is provided to adjust for the change in frequency characteristic of the signal as well as for the change in phase of the signal which are caused by the interposition of a magnetic circuit in the feedback loop. In the subtractor 21, the above-mentioned velocity output is negatively fed back and subtracted from the control signal. By the use of such a velocity feedback circuit, a differential component of the first order is added to the transfer characteristic of the linear motor 10 to thereby improve its response characteristic.
The above-mentioned velocity output can be measured by a circuit arrangement such as shown in FIG. 7. That is, the carriage 2 is made movable, and a driving current of a predetermined level is supplied from a variable frequency source 71 to the driving coil 11 to thereby move the carriage 2. The output of the velocity detecting coil 12 is amplified by he amplifier 23 with a predetermined gain.
The frequency characteristic of such a velocity output is shown in FIG. 9.
In FIG. 9, although the velocity output initially decreases with an increase of the frequency of the driving current, since the carriage 2 cannot continue to follow the increase, the decrease of the velocity output stops at about 20 Hz and, on the contrary, starts to increase when the frequency of the driving current exceeds about 100 Hz. Since the carriage 2 stops if the driving current frequency reaches about 10 to 20 Hz, the output of the velocity detecting coil 12 in a relatively high frequency band is noise dependent on the carriage velocity. The dotted line in FIG. 9 indicates a velocity output characteristic desirable for a feedback circuit.
FIG. 10 shows the phase change characteristic of the velocity output. The phase delay of the velocity relative to the driving current increases with increasing frequency of the driving current.
The velocity output is negatively fed back to the input side through the above-mentioned velocity equalizer 24. The characteristic at relatively low frequencies is shown in FIG. 9, and the phase change of the velocity output is large, as shown in FIG. 10.
With respect to stability surplus, that is, the gain surplus and the phase surplus of the feedback control system, which is an index of the servo operating range in which the feedback loop is prevented from oscillating, it is preferable to set the loop frequency, for example, not higher than 10 Hz. In this range, however, the band of the feedback loop is so narrow that a high degree of responsiveness is no longer possible.
In a video disk player or the like, on the other hand, in the case of special reproduction in which play is carried out in repetitive jumps over predetermined numbers of tracks, or in the case of so-called multi-jump in which jumping over a plurality of tracks is performed, good responsiveness is required in the operation of the carriage, and therefore it is desired to improve the above-mentioned velocity output characteristic.